This invention relates generally to antenna systems, and more particularly, to antenna systems used for communication to and from satellites. In many satellite communication systems, there is a need to provide high-gain independent beams covering an angular region of space. In such cases it is usually highly desirable to provide the multiple beams from a single antenna aperture, to minimize the size and complexity of the antenna system. It is also usually desirable in such multiple beam systems to provide low sidelobe radiation patterns, to minimize out-of-beam interference.
A fundamental problem arises when any attempt is made to provide multiple high-gain beams from a single antenna aperture, whether it be in the form of a lens or a reflector. The peak gains that one can achieve in the multiple beams are typically much lower that one can achieve using a single optimum antenna feed horn at the focal region of the lens or reflector. In terms of decibels (dB), the peak gain of each of the multiple beams may be 3 dB or more lower than that of an optimum single-feed antenna beam. The principal object of the present invention is to overcome this problem.
An important application of multibeam antenna systems is in communication to or from a synchronous earth satellite, i.e., one whose position is fixed relative to the rotation of the earth. A multibeam antenna system on such a satellite has to provide contiguous coverage of practically one hemisphere of the earth. The half-angle subtended by the earth at the position of a synchronous satellite is approximately 8.68 degrees. In configuring an array of beams to cover this angular area, there is a tradeoff between maintaining sufficient isolation between adjacent beams, and providing contiguous coverage at a sufficient gain over the entire angular area of the earth. It has been recognized that a desirable compromise is to provide contiguous coverage at a power of at least half the peak power of each beam. For this reason the half-power beamwidth (HPBW) of each beam is an important factor. The HPBW is the angular width of the beam measured at a point where the gain is one-half of the peak gain at the center of the beam. If adjacent beams, as defined by their half-power beamwidths, overlap sufficiently to leave no gaps, the array of beams is said to provide contiguous coverage at the -3 dB level, or half-power level, or better.
A conventional high-frequency antenna system includes an antenna feed horn through which transmitted signals are directed, and a focusing element, such as a reflector or lens, to focus the energy radiated from the feed horn into a beam. If some of the energy from the feed horn does not impinge on the focusing element, the system is clearly not operating at maximum efficiency. The gain of the antenna system is maximized when the primary radiation from the feed horn is practically all incident on the focusing element, and none "spills over" the edges of the element.
Unfortunately, there is a fundamental disparity between the feed horn aperture size required to maximize gain and the feed horn size necessary to permit packing the beams at half-power beam width spacing. Specifically, it can be shown that the feed horn diameter to maximize gain is substantially larger than the feed horn diameter that will permit close packing, i.e. with coverage to the -3 dB level, in a single conventional focusing reflector or lens. Accordingly, horn aperture sizes that yield maximum gain lead to beam separations much larger than one half-power beamwidth.
For a single feed horn of given diameter, maximum gain is yielded by an optimum value of the reflector or lens included angle, i.e. the angle subtended by the reflector or lens at its focal region. However, if multiple feed horns of the same given diameter are placed side by side and used with the same reflector or lens arrangement, the resulting beams will be spaced from each other by much more than the desired 3 dB crossover. If one then makes either the horn feed diameters smaller or the included angle smaller, until the desired 3 dB crossover is obtained, some of the energy from the feed horns does not impinge on the reflector or lens. This "spillover" loss substantially reduces the overall efficiency of the antenna system. Furthermore, this limitation of conventional reflector and lens systems is independent of the focal length to aperture diameter ratio (F/D) of the reflector or lens.
In summary, for a given feed horn aperture in a focused antenna system of the prior art the only way to achieve a desired beam overlap for a given beamwidth is to vary the included angle of the lens or reflector of the system. For a desirable beam spacing at the -3 dB level, the lens or reflector included angle has to be reduced below its optimum value, and then there is "spillover" loss and lowered efficiency.
A possible solution to this problem is to provide multiple lenses or reflectors. Then each lens or reflector does not have to accommodate multiple beams in such a closely spaced relationship. However, the multiple lenses or reflectors introduce additional complexity, and must be maintained is very precise alignment for good results. It will be appreciated from the foregoing that there is a need for a single-reflector or singlelens multibeam antenna system capable of providing closely packed, secondary beams, but without degradation of the efficiency of the system. The present invention fulfills this need.